0

I have problem of understanding fully "isolated system", if i imagine a car moving at a constant speed ,can I say that is isolated system although an interna force (car's engine) cancel out an external force ( friction)

Gio lou
  • 11
  • Well isolated just means that nothing outside of it affects it. So your car is not isolated if there is friction with the road, but if the road were friction-less or gravity was off, then it would be isolated despite that internal forces on the engine and gears, etc. – rmhleo Jan 16 '16 at 22:12

2 Answers2

1

An isolated system is just a way of saying energy is conserved within the system. That is, a system which is not isolated can transfer energy into its surroundings or have energy transferred into it from the environment. In your example of the car, friction between the road and the tires is a way in which energy can be lost to heat up the road, so the car is not isolated. However, the engine and gasoline are part of the car, but the fact that the car touches the road does not noticeably change the process of extracting chemical energy from the gasoline, so it would make sense to model that part of the physics as a closed system (perhaps this is simpler if we consider an electric car).

Of course, there is no such thing as a truly isolated system. It is a limiting case which is useful in situations where you're studying something which does not exchange energy with the environment on the scale of the dynamics you wish to study.

CasualScience
  • 2,027
0

An isolated system can experience internal forces like from the car's engine. It is no longer an isolated system when you count friction in, which is an external force on the system.

Friction is namely the force that pulls the car forward to make it accelerate. The system consisting of the car can only accelerate when experiencing a non-zero net external force. And the net force would consist of just friction in such case.

If the car does not accelerate but has constant speed as you mention, then there is no external net force! This is simply Newton's first law. And if there are no other horizontal forces possible than friction (neglecting air drag etc), then friction must be zero - which it precisely is when the car drives at constant speed. And now the car is an isolated system again, since no net external forces work on it.

Steeven
  • 50,707
  • So I can use conservation of momentum...?? – Gio lou Jan 16 '16 at 23:59
  • If it's an isolated system, then yes. So, if it's a car driving at constant speed and a horizontal road, then yes. If it hits something, conservation of momentum us valid. – Steeven Jan 17 '16 at 00:00
  • If I suppose that a mass m collide vertically with this car.(dropped from the sky)...I can say Mc(V)=(Mc+m)V'...Mc=car's Mass...??????.but if I do this...isn't there a problem with friction??Because friction will increase slowing down the car.... – Gio lou Jan 17 '16 at 00:11